In a charged-particle microscope (CPM), an imaging beam of charged particles is directed onto a sample from an illuminator. In a transmission-type CPM (TCPM), a detector is used to intercept a flux of charged particles that traverse the sample, generally with the aid of an imaging system that is used to focus (part of) said flux onto the detector. Such a TCPM can be used in scanning mode (STCPM), in which case the beam of charged particles from the illuminator is scanned across the sample, and the detector output is recorded as a function of scan position. In addition to imaging, a CPM may also have other functionalities, such as performing spectroscopy, examining diffractograms, performing (localized) surface modification (e.g. milling, etching, and deposition), etc. An illuminator refers to a particle-optical column comprising one or more electrostatic and/or magnetic lenses that can be used to manipulate a “raw” charged-particle beam from a source (e.g. a Schottky source or ion gun), serving to provide it with a certain focus or deflection and/or to mitigate one or more aberrations therein. An illuminator can be provided with a deflector system that can be invoked to cause the beam to perform a scanning motion across the sample under investigation.
Electrons, because of their wave-particle duality, can be accelerated to have picometer wavelength and focused to image in real space. It is now possible to image with high resolution, reaching the sub-Angstrom scale. Well-known electron microscopes include Transmission Electron Microscope (TEM), Scanning Electron Microscope (SEM), Scanning Transmission Electron Microscope (STEM), and “dual-beam” tools (e.g. a FIB-SEM), which additionally employ a “machining” Focused Ion Beam (FIB), allowing supportive activities such as ion-beam milling or Ion-Beam-Induced Deposition (IBID). In a TEM, the electron beam used to irradiate a sample will generally be of significantly higher energy than in the case of a SEM (e.g. 300 keV vs. 10 keV), so as to allow its constituent electrons to penetrate the full depth of the sample. A sample investigated in a TEM will also generally need to be thinner than that investigated in a SEM. In traditional electron microscopes, the imaging beam is “on” for an extended period of time during a given imaging capture; however, electron microscopes are also available in which imaging occurs on the basis of a relatively short “flash” or “burst” of electrons, which approach is particularly useful when a user is attempting to image moving samples or radiation-sensitive specimens.
Scanning transmission electron microscope (STEM) is operated under a principle similar to scanning electron microscope (SEM). A primary beam is emitted from an electron source and focused by objective lens on a specimen which is about 100 nm in thickness. Deflectors drive the focused primary beam to scan on the specimen, and the electrons that have penetrated through the specimen are collected by a detector to generate an image. FIG. 1 schematically illustrates the configuration of a conventional scanning transmission electron microscope in the prior art.
With reference to FIG. 1, the scanning transmission electron microscope 100 comprises an electron source 101 for generating a primary electron beam 102a onto a specimen 106 along center optical axis 103. In a high resolution STEM, the electron source is generally a thermal field emission electron source or a cold field emission electron source. The STEM 100 also includes an objective lens 105 for forming the magnetic field to focus the primary electron beam 102a onto the filmy specimen 106. Condenser lens, image aperture and other optical components which are located between the electron source and the objective lens are not shown in FIG. 1. A deflection system 104 for deflecting the primary electron beam 102a over the specimen 106 to form a scanning pattern. The deflection system consists of two magnetic or electrostatic deflectors 104a and 104b which are away from objective lens field. The specimen 106 is put on a stage for adjusting the specimen height to the focused plane of primary electron beam and moving observed area of specimen. The specimen 106 is between the upper pole 105a piece and lower pole piece 105b of objective lens to make an immersion objective lens system. Immersion objective lens ensures a small focused spot on axis because the spherical aberration coefficient and chromatic aberration coefficient are smaller than the objective lens field far away from the specimen. The focused field above specimen 106 can be shown as a convex upper lens field 107; and the focused field below the specimen 106 can be shown as a convex lower lens field 108.
Primary beam landing on filmy specimen is focused by objective lens 105. Then transmission electron beam 109a is formed and received by a transmission electron detector 110. The transmission detector includes a bright-field detective area 110b in center to only catch transmit electron through specimen 106 and a circular dark-field detective area 110a outside to catch the scatter in and transmit electron through specimen 105. Sometimes bright-field detective area 110a and dark-field detective area 110b are located on different height to enhance receiving efficiency. As shown in FIG. 1, in scanning center, deflection system does not work on the primary beam. The transmission electron beam is perfectly caught by bright-field area and dark-field area following distribution of transmission angle.
FIG. 2 shows a modified STEM 100a based on conventional STEM 100 as shown in FIG. 1. When the primary beam 102b is deflected to off-axis position in scanning field expect the scanning center, deflector 104a deflects the primary beam first, and then deflector 104b deflects the beam back to a region close to the optical axis of objective lens. The rational is not to make a large off-axis aberration in scanning field edge, but the scanning field is limited because primary beam 102b cannot be deflected into a region far away from the center optical axis 103. Moreover, the transmission electron beam 109b cannot be received by detector as the center transmission electron beam 109a. Bright-field detective 110a area cannot catch the pure direct transmission electrons, and dark-field detective area 110b cannot perfectly catch the scatter transmission electrons from large radical emission angle. As such, image quality at the scanning field edge is not as good as that in the center.
FIG. 3 shows a modified STEM 100b based on STEM 100a as shown in FIG. 2. An additional de-scan deflective system 111 is installed below the specimen and away from objective lens to correct the transmission electron beam from scanning field back to the center optical axis. This de-scan deflective system has one or two magnetic or electrostatic deflectors, 111a and 111b. De-scan deflective system 111 deflects transmission electron beam 109c from off-axis position back to center optical axis. So the transmission electron beam from different scanning position will have the same or similar radial emission angle to be projected on the detector. De-scan deflective system 111 eliminates the position effect on transmission electrons reception. But the scanning field of primary beam remains limited by the deflective system 104 and objective lens 105, especially on high resolution image condition. If the deflective system deflects primary beam at a large scanning field edge, off-axis aberrations will increase drastically because primary beam dose not enter into the region near axis of focused field 107 above specimen. On the other hand, de-scan deflective system cannot correct transmission electron from the position with large radial distance position. At the position with large radial distance from center optical axis, transmission electrons through specimen enter into the region far away from the axis of focused field 108 below specimen. The transmission electron beam is converged strongly by focused field 108 with large off-axis aberrations and the projection trajectory is very different from the center transmission electron. The de-scan deflective system cannot correct the trajectory of transmission electrons from large scanning field edge and center transmission electrons back to center optical axis simultaneously.
In the prior art, the scanning field is limited below several micro meters (um) at high resolution image mode. For example, conventional STEM usually has 0.5 um×0.5 um maximum scanning field with 0.5 nm resolution, or 2 um×2 um maximum scanning field with 2 nm resolution.
In high resolution STEM, primary electron beam is intensely focused on specimen. The focus spot size is usually in the magnitude of several nanometers, or even several angstroms. Usually, a specimen is placed in the focus field of the objective lens (in-lens type). This design of lens shows relatively small aberration coefficients. U.S. Pat. Nos. 7,285,776, 7,355,177, 7,459,683 and 7,745,787 disclose an in-lens type objective lens for STEM. Thin specimen is placed between upper pole piece and lower piece of objective lens, and the specimen is immersed in the magnetic field of the objective lens. The in-lens type structure ensures a small spot size on specimen, in other words, it ensures a high resolution image.
The size of the scanning field has a great impact on the throughput of STEM. At the same scanning speed and beam current, the larger the scanning field, the higher the throughput. Compared to STEM with large scanning field, STEM with small scanning field demands more time in manipulating the specimen stage (e.g. moving and stopping) in order to change the area of interest (or observed portion) in the specimen under inspection. Mechanical manipulation of specimen stage takes much longer time than electron beam scanning.
In traditional in-lens objective lens structure, specimen is immersed in focus field to ensure high resolution, but the scanning field is very small. Since the focus magnetic lens is very close to specimen, at the short focal length, the off-axis aberrations such as coma, chromatic aberrations and distortion increase quickly, and in proportional to the distance from center optical axis. To achieve approximate resolution between center and scanning field edge, the primary beam must be deflected only near the optical axis, and thus cover a small scanning field. Traditional in-lens type STEMs usually have several micrometers or several hundred nanometers scanning field at high resolution (several nanometers or several angstroms spot size).
U.S. Pat. No. 4,544,846 teaches a variable axis immersion electron lens (VAIL) projection system. A deflector having a designed field coupled to focus field can shift the optical axis of objective lens. When the axis is shifted to the same position and direction as the scanning beam, the off-axis aberrations are eliminated, and a small spot size is obtained similar to that with the center optical axis.
U.S. Pat. No. 6,392,231 discloses another VAIL system, called swing axis immersion electron lens (SAIL). The SAIL is used to achieve a large scanning field in SEM. A deflector having a designed field coupled to focus field can swing the optical axis of objective lens. When the axis is swing to the same position and direction as the scanning beam, the off-axis aberrations are eliminated and a similar spot size is obtained as center optical axis can.
Yan Zhao et al. have attempted to use the variable axis objective lens concept in SEM and electron beam lithography. They have also proposed on how to make different types of variable axis system by using different types of coupling conditions between deflectors and objective lens. For details, see Yan Zhao et al. “Comparative study on magnetic variable axis lenses using electrostatic and magnetic in-lens deflectors”, Proceedings of the SPIE, Volume 3777, 1999, p. 107-114; as well as Yan Zhao et al. “Variable axis lens of mixed electrostatic and magnetic fields and its application in electron-beam lithography systems” Journal of Vacuum Science & Technology, B 17(6), Nov/Dec 1999, p. 2795-2798.
When the primary beam is focused on a specimen in an immersion field, electrons that transmit the thin film specimen is focused and projected on a detector. The transmission electrons carry the information about the structure and materials contrast of specimen. The detector is usually divided into bright-field detective portion and dark-field detective portion to catch the transmission electron with different angular ranges. The bright-field detector collects the direct transmission electrons and the dark-field detector mainly collects the scatter transmission electrons taking atomic number signal in large angular range. In STEM, the transmission electrons from the scanning field edge have a different position on detector compared to center transmission electrons on optical axis. Thus the transmission electrons from scanning field except the center cannot be projected on detector circle symmetrically as the center transmission electrons. The bright-field and dark-field detectors cannot catch the signal transmission electrons precisely corresponding to transmission angle, so the image quality and contrast at the edge of the scanning field is worse than scanning center. It is impossible to obtain a high resolution image in large scanning field. To resolve the problem, international application publication WO2012/009543 discloses a de-scan deflective system that is put below the specimen in a STEM to correct the projection trajectory of transmission electrons. This de-scan deflective system eliminate the difference of projective location of transmission electrons on detector. But compared with the deflectors which are coupled with focus field in variable axis lens system, the deflector in WO2012/009543 can only correct the transmission electrons from relative small scanning field.
Thus, there is a need to enlarge the scanning field of STEM, and in the meanwhile, to maintain a high resolution image. For example, a trend in recent years is using an electron microscope to generate high resolution image for 3D reconstruction of tissue volume in biological research. However, the throughput of convectional scanning transmission electron microscope is not satisfactory, because of the small scanning field and low scanning speed. Advantageously, the STEM of the invention can acquire high resolution images in large area and at a high speed.